The basic function of a condom is to prevent the transmission of semen. By stopping that one thing, you can prevent both pregnancy and the spread of tons of STIs during intercourse.

"Condoms used properly and consistently are effective in preventing transmission of HIV, hepatitis B, hepatitis C, syphilis, gonorrhea, and chlamydia,"Fahimeh Sasan, D.O., assistant professor of obstetrics, gynecology, and reproductive science at the Icahn School of Medicine at Mount Sinai, tells SELF. That's because latex is an effective barrier that's impermeable to STI pathogens, the CDC says. (Note: Lambskin condoms are effective for preventing pregnancy but should not be used for STI prevention, as the small particles are able to penetrate them.)

The problem is that not all STIs are spread through semen, meaning a condom may not be the magic shield you thought it was. "Anything that can be spread with skin-to-skin contact is not going to be protected for by a condom," Tami Rowen, M.D., an ob/gyn at UCSF Medical Center specializing in sexual health concerns, tells SELF.

Here are the STIs that can still be spread even when your partner has things under wraps, and why it's so important to get tested regularly if you're sexually active.

1. Herpes

"If a person has a herpes lesion that is on an area of the groin or pelvis that is not covered by the condom, then transmission from that lesion can occur," Sasan explains. Doctors recommend abstaining when you have an active outbreak (that's when you're most infectious) to decrease the risk of spreading the virus to your partner, though it can still be transmitted when there's no visible outbreak, too.

2. HPV

HPV is the most common STI—if you're sexually active, chances are you'll be exposed to it at some point in your life. Some strains can cause genital warts but others don't, which is why many people never know they have it. And even without warts present, HPV can be passed with skin-to-skin contact. Some strains can lead to cervical cancer, and while there's evidence that condom use reduces cervical cancer risk, there isn't direct evidence of its ability to prevent specific HPV strains, both "dangerous" and not.

3. Molluscum

Molluscum is a virus that manifests as tiny little bumps on the skin. It's not a very familiar STI, because it doesn't cause any long-term health problems and is typically asymptomatic other than the appearance of bumps. "It’s a skin condition commonly transmitted between kids, but in adults you usually see it sexually transmitted," Rowen says. Women will typically notice it on the vulva. The small, round bumps can appear anywhere from two to three months after the time of infection, and may itch or feel tender to the touch.

4. Pubic lice (crabs)

"Pubic lice is not super common anymore, since people groom more now," Rowen explains. But if you're rocking a full bush, you do have a chance of catching pubic lice from an infected partner. Since lice lay eggs and live in the hair, a condom will do diddly squat to keep you safe.

5. Syphilis

Syphilis presents as a firm, round, and usually painless sore, called a chancre. "Just like herpes, [a condom] is not going to protect you, depending on where the sore is located," Rowen says. Wearing a condom decreases the likelihood it will spread, and can be effective if the chancre is covered by the condom, but if it is located on a part of the penis that's exposed, it can be transmitted.

Bottom line: If you’re sexually active, it’s important to get tested regularly even if you practice safe sex.

If you have multiple partners—or if your partner might have multiple partners—it's absolutely crucial that you get tested regularly. This is particularly important given the fact that “many men and women are asymptomatic when they have a sexually transmitted infection,” Sasan says. She recommends getting tested every three to six months—or, before "initiating sexual contact with a new partner." (You should also ask any new partners when they were last tested, to make sure they've got a clean bill of sexual health, too.)

Getting tested regularly isn't just about avoiding passing STIs back and forth, but it's also important to prevent an infection from having a greater impact on your health. Some STIs, if left untreated for too long, can lead to infertility, so it's better to catch one early so you can treat it right away.